This invention relates to a control system for controlling the working depth of a tractor-coupled implement as a function of various sensed and operator-controlled parameters.
Most currently available hydromechanical implement control systems operate to control the working depth of the implement purely as a function of the implement position, purely as a function of the draft force sensed in the hitch linkages or as a mixed function of the sensed implement position and sensed draft force. Such control systems are limited in that they do not prevent excessive wheel slippage nor do they prevent engine stalling during certain adverse conditions. For example, with draft sensing only, the conventional draft force responsive control system may cause the engine to be lugged down and possibly stalled, due to inaccurate draft sensing, increased rolling resistance or burrowing of the drive wheels. It has been proposed, as disclosed in U.S. Pat. Nos. 2,927,649; 3,776,322; 3,834,481 and 3,913,680, to control the implement working depth purely as a function of the amount of slippage of the drive wheels, thereby using wheel slippage input as a substitute for a sensed draft force input. Of course, such systems do not provide the advantage of draft force control, nor do they necessarily prevent engine stalling. A rotary implement control system, described in U.S. Pat. No. 4,077,475, includes multiple independent control modes which are individually selected by a selector switch. These control modes include a sensed draft force only control mode, a sensed position only control mode, a manual control mode and a mixed control mode wherein the implement working depth is controlled in response to a mixture of hitch position and engine speed. Another control system is described in U.S. Pat. No. 3,860,074, wherein the control signal is the most positive of a draft error voltage and a position error voltage. However, these control systems do not provide protection against excessive wheel slippage in any of their modes. Furthermore, none of these control systems provide for hitch or implement control simultaneously as a function of sensed wheel slippage combined with other sensed parameters, such as sensed draft force or sensed engine speed. Finally, none of these control systems provide for automatically shifting or switching between one control mode, such as a pure position control mode, and a combined control mode wherein the hitch is controlled as a function of a combined signal representing sensed draft force and sensed wheel slip and engine speed.